The invention relates to screens, and especially to generally tubular, but non-cylindrical, hollow screen members which include an elongated, curved surface which is perforated, slotted, or has some other form of partially open filtering surface, and which has sufficient strength to resist the external forces which might be applied to it, such as by a bed of particulate material, for example. One prior art example of such a screen, which is adapted to be used in a vertical arrangement inside a reactor vessel, comprises a plurality of scallop elements which are adapted to be arranged around the inside of the outer wall of the reactor vessel. In such a vessel, a generally annular bed of catalyst might be positioned between the radially inwardly facing partially open surfaces of the plurality of scallops and a cylindrical, perforated inner screen member. The scallop members and the inner screen are collectively referred to as the "internals" of the reactor. In operation, a reactant gas entering, for example, the open upper ends of the scallops, would first pass radially inwardly through the openings formed in the inner wall surfaces of the scallops and would then pass radially inwardly through the catalyst, through the perforated outer surface of the inner screen member, and then exit through the bottom end of the inner screen member. Although the fluid in a reactor type of scallops installation is typically a gas, it could also be a liquid.
Typically, screen members of the type hereinbefore described are produced by punching a large plurality of small oblong slots into a sheet of metal which is then formed and welded into the desired generally tubular shape. The perforations are usually limited to one side of the screen, with the opposite side being unperforated. Although strength requirements often dictate a certain minimum thickness of metal be used for a screen, there appear to be certain physical limits as to how narrow a slot can be punched in a sheet of a given thickness. For example, it has been found to be quite difficult from a technological standpoint to produce slots narrower than about 0.042" in a sheet of stainless steel having a thickness of about 0.048". Also, the open area of such a screen is relatively low, about 26%, as compared to some other types of screens such as, for example, the cylindrical water well screens shown in U.S. Pat. No. 2,046,458 issued to E. E. Johnson. The type of screen shown in the aforementioned Johnson patent is typically produced by helically wrapping an elongated strand of wire having a wedge or keystone-shaped cross-section around the circumference of a plurality of circumferentially spaced, longitudinally extending internal support rods. The wrap wire is typically welded at every intersection with said rods to establish the spacing or width of the slots which are formed between adjacent wraps of wire. The slot spacing of the aforementioned wrapped welded wire screens, and thus the open area of the screen, can be readily varied by varying the rate at which the rods are advanced for each revolution of the cylinder.
Screens of the aforementioned wrapped wire type are preferably formed as a cylinder, whereas scallops must typically be formed so as to have a non-cylindrical shape including a pair of opposed, outwardly convex surfaces. Although it has been possible, in the manufacture of scallops, to take advantage of the finer slot sizes, which can be as small as 0.001", and greater open area which are available with wrapped wire screens, it has been necessary to use quite expensive fabricating techniques in order to achieve a satisfactory scallop assembly. For example, to produce such a scallop assembly, a wrapped wire screen cylinder is longitudinally split and then flattened sufficiently to produce a curved panel of the desired radius of curvature for the perforated side of the scallop assembly. A flat sheet of metal, usually stainless steel, is then curved to the desired radius of the back surface of the scallop and its two side edges are bent toward each other rather sharply so as to project forwardly in the curved plane of the partially flattened slotted screen panel. The metal sheet is then welded along its side edges to the screen panel so as to produce a composite screen assembly in which the bent side edges of the metal sheet overlap the edges of the screen panel. The resulting screen assembly is generally satisfactory but is far more costly to produce than a scallop assembly formed solely from a perforated sheet.
During normal, low pressure drop operation of a particular catalyst regenerating vessel, perforated plate type scallops are generally sufficient. However, heat-up and cool-down of the vessel can cause significant short-term loading on the internals, including the scallops. High loadings can also be generated by operation beyond normal regeneration requirements, fouling of the catalyst bed, high local temperatures and carbon clinker growth within the bed. These severe conditions require stronger internals with design variability to meet the operating conditions seen in specific units.
Perforated scallops in reformer service have open area and strength limitations. As previously noted, the sheet thickness of the formed metal scallop is limited by the size and number of perforations which it is physically practical to punch in the sheet. Openings in the form of 0.042".times.1/2" oblong slots in 0.048" thick material is stretching the normal industry practice of minimum opening size equal to material thickness. A punched hole size of 0.042" is generally considered to be the largest sized opening that can be used to safely retain a conventional 0.062" diameter catalyst. This maximum width opening effectively defines the maximum thickness of material used to make the scallop and, therefore, the greatest strength for the scallop. Almost no alternatives to this sheet thickness/perforation size limitation are available within punched sheet technology. Thus, a punched sheet scallop is limited in collapse strength, open area, and vertical load component strength because the sheet thickness is defined. Attempts at reinforcing perforated scallops have been made via internal stiffeners and heavier gauge backs welded to 0.048" thick fronts. The use of such stiffeners does not, however, necessarily increase the strength of the scallop. This is because the loading imposed on the scallop face by a catalyst bed is typically not in uniform hydraulic directions. Rather, the granular bed imposes more of a straight line load. Under high loading, the back surface of a scallop will initially straighten out sufficiently that it will seat completely on the curved vessel wall. At that time, a classic arch is formed, which, under load, fails in buckling at a definable point. Adding a vertical stiffening rib at the center actually reduces the collapse strength by inducing the buckling mode. The stiffener does not allow a true arch to form, thus severely weakening the ability of the scallop to resist collapse. The stiffener does, however, increase longitudinal stiffness for ease of handling.
In addition to the aforementioned cylindrical and flattened screen members which have been discussed as having been used in vertical arrangements, there are various types of screens which have been used in horizontal arrangements, such as underdrains for gravity or pressure filters. Some of these, such as the arrangement shown in Kastler U.S. Pat. No. 3,247,971, include quite wide slots which can only be used with a coarse filter media, or which would require successive layers of media having various degrees of coarseness. A generally flat arrangement, such as disclosed in Emrie U.S. Pat. No. 4,331,542 must be attached to the floor of the filter chamber. Evans U.S. Pat. No. 4,098,695 and Novotny U.S. Pat. No. 4,098,695 each show horizontal distributor/collector arrangements having cylindrical screen laterals with internal piping which has a smaller open flow area than that of the screen surface so that back pressures can be provided during backwashing that will assure relatively uniform distribution of the backwashing fluid to those portions of the media bed which are above the laterals. Because the laterals in arrangements such as those shown by U.S. Pat. No. 4,013,556 and 4,331,542 are somewhat widely spaced from each other, it is obvious that a backwashing operation will not be able to contact the media which is located near the bottom of the filter bed and between the laterals as well as it does the media immediately above a lateral. One could consider greatly multiplying the number of laterals but this would not generally be feasible from an economic standpoint.